The present invention relates to a ferroelectric liquid crystal display such as a liquid crystal display panel or a liquid crystal light shutter array having a liquid crystal layer of ferroelectric liquid crystal.
Generally, ferroelectric liquid crystal molecules are known to move along the side surface of a cone (hereinafter referred to as xe2x80x9cthe liquid crystal conexe2x80x9d) with the change of an external object such as the electric field. In the case where the ferroelectric liquid crystal is held between a pair of substrates and used as a liquid crystal panel, the ferroelectric liquid crystal is controlled in such a manner that the ferroelectric liquid crystal molecules are located at one of the two places on the side surface of the liquid crystal cone. The stable state in which the liquid crystal molecules are located at one of the two places is called a first ferroelectric state or a second ferroelectric state, as the case may be.
FIG. 1 is a diagram showing an example of the configuration of a ferroelectric liquid crystal panel when the ferroelectric liquid crystal is used as a display. A liquid crystal cell 2 is arranged between polarization plates 1a, 1b conforming to a crossed Nicol in such a manner that either the polarization axis a of the polarization plate 1a or the polarization axis b of the polarization plate 1b is parallel to the direction along the long axis of the molecules in the first ferroelectric state or the second ferroelectric state when a voltage is not applied. In the case of FIG. 1, the direction along the long axis of the molecules in the second ferroelectric state coincides with the polarization axis a.
In the case where the polarization plate is arranged as shown in FIG. 1, the light is not transmitted and black is displayed, on the ferroelectric liquid crystal panel, when the ferroelectric liquid crystal is in the ferroelectric state in which the direction along the long axis of the molecules coincides with the direction along the polarization axis of the polarization plate. With the configuration shown in FIG. 1, the light is not transmitted and black is displayed (non-transmission state), on the ferroelectric liquid crystal panel, when the ferroelectric liquid crystal is in the second ferroelectric state.
With the change in polarity of the applied voltage, on the other hand, the ferroelectric liquid crystal comes to assume a ferroelectric state with the direction along the long axis of the molecules thereof failing to coincide with the direction along the polarization axis of the polarization plate. In such a case, the molecules of the ferroelectric liquid crystal are slanted at an angle to the polarization axis, and therefore the light from the backlight is transmitted and white is displayed (transmission state).
In FIG. 1, the direction along the long axis of the molecules when the ferroelectric liquid crystal is in the second ferroelectric state is rendered to coincide with the direction along the polarization axis of the polarization plate. On the other hand, the direction along the long axis of the molecules when the ferroelectric liquid crystal is in the first ferroelectric state can be rendered to coincide with the direction along the polarization axis of the polarization plate. In such a case, black can be displayed (non-transmission state) when the ferroelectric liquid crystal is in the first ferroelectric state, and white can be displayed (transmission state) when the ferroelectric liquid crystal is in the second ferroelectric state.
The present invention is applicable to either panel configuration. In the following description, the case where the panel configuration shown in FIG. 1 is employed will be explained.
When a voltage is applied to this ferroelectric liquid crystal panel, the change of the light transmittance plotted as a graph follows a loop as shown in FIG. 2.
The switching of the ferroelectric-liquid crystal, i.e. the transition from one ferroelectric state to the other ferroelectric state occurs only in the case where a voltage with the value of the product of the wave width value and the wave height value, which is not less than the a threshold value, is applied to the ferroelectric liquid crystal molecules. As shown in FIG. 2, either the first ferroelectric state (transmission, white display) or the second ferroelectric state (non-transmission, black display) is selected according to the difference in polarity of the applied voltage.
The voltage value at which the light transmittance begins to change, when the voltage is applied and increased, is denoted by V1, the voltage value at which the change of the light transmittance is saturated is denoted by V2, on the other hand, when the voltage is decreased and further the voltage of opposite polarity is applied, the voltage value at which the light transmittance begins to decrease is denoted by V3, and the voltage value at which the change of the light transmittance is saturated is denoted by V4.
As shown in FIG. 2, in the case where the value of the voltage applied is not less than the threshold value of the ferroelectric liquid crystal molecules, the first ferroelectric state (transmission, white display) is selected. In the case where a voltage of opposite polarity not less than the threshold value of the ferroelectric liquid crystal molecules is applied, on the other hand, the second ferroelectric state (non-transmission, black display) is selected.
A typical driving waveform of a ferroelectric liquid crystal display using a ferroelectric liquid crystal panel with the polarization plate arranged as shown in FIG. 1 is shown in FIG. 3. In the drawing, (a) designates a scanning voltage waveform (b) a signal voltage waveform, (c) a combined voltage waveform, and (d) a light transmittance.
A time division driving method is known as a method of driving a ferroelectric liquid crystal display. In the time division driving method, a plurality of scanning electrodes and signal electrodes are formed on a substrate and a voltage is applied to each electrode to drive a liquid crystal element.
As shown in FIG. 3, the writing operation is performed by applying a scanning voltage (a) to the scanning electrodes, a signal voltage (b) to the signal electrodes and a combined voltage (c) to the pixels of the liquid crystal panel. FIG. 3 shows driving waveforms of two frames, in which ON indicates the white display and OFF the black display. The driving waveforms shown in FIG. 3 have one scanning period for carrying out the display based on one display data. One scanning period has therein a selection period (Se) for selecting the display state and a non-selection period (NSe) for holding the selected display state, and a reset period (Rs) is provided before the start of the selection period (Se).
To write the next display, the ferroelectric liquid crystal is reset to one ferroelectric state in the reset period (Rs) regardless of the immediately preceding display state. In the case of FIG. 3, in the first half of the reset period (RS), the ferroelectric liquid crystal is reset to the first ferroelectric state in which the liquid crystal is white display (transmission state) and, in the last half, is reset to the second ferroelectric state in which the liquid crystal is black display (non-transmission state). In this method of driving the ferroelectric liquid crystal display, it is common to provide a reset period for applying a pulse of a different polarity regardless of the immediately preceding display state in order to assure a satisfactory display.
Conventionally, in the case where an image with a rapidly changing screen is displayed on a liquid crystal display unit using an ordinary liquid crystal such as nematic one, a phenomenon has occurred in which an image accurately following the screen change cannot be displayed. In a typical case where an image of a ball moving about in a game is displayed on a liquid crystal display unit, a phenomenon may occur in which the contour of the ball cannot be displayed accurately but displayed blurred (hereafter, this phenomenon is referred to as the xe2x80x9ctrailing phenomenonxe2x80x9d). Conventionally, this trailing phenomenon was considered to occur due to the slow switching of the liquid crystal molecules. In recent years, however, it has been reported that this phenomenon occurs not only for the slow switching of the liquid crystal molecules, but also for the conventional driving method of the liquid crystal. The reason is that, in the scanning period in which the display is written in the pixels based on the display data, when writing the next display in the pixels without reset the display preceding the writing, the preceding display is left as a persisting image in the human eyes viewing the display, thereby causing the trailing phenomenon.
The ferroelectric liquid crystal has so far been considered to cause the trailing phenomenon not easily due to its rapid switching characteristic as compared with an ordinary liquid crystal. The recent research efforts have revealed, however, that in addition to the rapid switching characteristic, the driving method unique to the ferroelectric liquid crystal, i.e. the provision of a reset period described above is another contributing factor for a reduced trailing phenomenon.
Nevertheless, the past research efforts have not clarified the manner in which the ferroelectric liquid crystal is to be controlled most effectively during the reset period to reduce the trailing phenomenon. Further, the problem still remains unsolved that the mere provision of a reset period cannot completely eliminate the trailing phenomenon in the animation display in which display data changes at high speed successively.
In view of this, the object of the present invention is to provide a ferroelectric liquid crystal display using a ferroelectric liquid crystal having a superior display quality, in which an optimum reset period is provided for displaying an animation or a still image.
In order to achieve the above-mentioned object, a ferroelectric liquid crystal display according to this invention is characterized in that at least one scanning period is set for executing the display based on one display data, a reset period is provided for resetting the ferroelectric liquid crystal to the black display state before starting the scanning period, and the length of the reset period is adjusted in accordance with the display data, thereby providing an optimum reset period corresponding to the display data.
In this invention, the reset period is provided for each pixel. Also, the reset periods can be provided at the same timing to reset all the pixels at the same time.
The ferroelectric liquid crystal panel has a backlight, which is turned off during the reset period and turned on during the remaining period. The ferroelectric liquid crystal panel also has a mechanism for adjusting the brightness of the backlight in accordance with the length of the reset period.
In the reset period, the ferroelectric liquid crystal has a first ferroelectric state and a second ferroelectric state, and the scanning period has a selection period (Se) for selecting a display state and a non-selection period (NSe) for holding the selected display state.
The mechanism for adjusting the length of the reset period is a control device for adjusting the reset period and can be adjusted manually while watching the screen. The mechanism for adjusting the length of the reset period, on the other hand, has a plurality of display data memories for continuously storing the display data, or sequentially storing the display data at intervals of a plurality of blocks into which the display data are divided, and a display data comparison circuit for comparing the display data in the display data memories with each other and outputting the result of comparison, i.e. the change amount of the display data. The reset period can thus be automatically adjusted according to this output.
With the ferroelectric liquid crystal display according to this invention, a reset period is provided in which black is displayed regardless of the display state of the ferroelectric liquid crystal, and the length of the reset period is adjusted according to the change amount of display data, i.e. whether an animation or a still image is involved, thereby making it possible to produce a superior display quality free of the trailing phenomenon. Also, the brightness of the backlight is adjusted in accordance with the length of the reset period. Therefore, a satisfactory display can be obtained by maintaining a certain brightness, both in the case of a display slow in motion such as a still image and in the case of a display rapid in motion such as a game.
The ferroelectric liquid crystal display according to the invention, which is described as an invention for display, can be used as a liquid crystal shutter. In such a case, the driving waveforms described as related to an animation and a still image correspond to the driving waveforms for high shutter speed and low shutter speed, respectively. By using these driving waveforms, therefore, the present invention can be applied to the liquid crystal shutter.